fix: address Copilot review — const precedence, TUI variant model, decline both-keyword unions

- rootUnion: decline a schema carrying BOTH oneOf and anyOf rather than
  reading one and silently dropping the other's constraints.
- SchemaForm: handle `const` before every type dispatch and for every
  constant type, so a numeric/boolean constant, or a schema carrying both
  `const` and `enum`, can no longer offer a forbidden value.
- collectSchemaDefaults / TUI initialValue: `const` outranks `default`,
  which is an annotation a schema may set to something its const rejects.
- convertToolParameters: resolve from the first branch's merged schema, so a
  branch's specialization of a root-declared property is what types the arg.
- TUI: ink-form keys values by field name across the whole form, so two
  branches' `kind` were one field and the later section's initial value
  decided what the earlier submitted. Branch fields now render under prefixed
  names behind a Variant select, and decodeFormValues translates back on
  submit, dropping every branch but the chosen one.

Signed-off-by: cliffhall <cliff@futurescale.com>

README.md

@@ -300,11 +300,13 @@ Open the Tools tab and select `echo`. Above the fields is a **Variant** picker l
 
 Switching branches drops the values that belonged to the outgoing one. They are no longer on screen, so the user can neither see nor clear them, and submitting them would describe a shape the call is not making.
 
-The **TUI** has the same gap and is worth checking against the same server (`--tui`, then test `echo`). ink-form is static — there is no picker to hide the alternatives behind — so each branch becomes its own **section**, and the fields in it are rendered optional whatever the branch says: only one alternative applies to a call, so requiring them would build a form that can never be submitted. Untouched fields report no value and are dropped before the call, so the sections you skip contribute nothing.
+The **TUI** has the same gap and is worth checking against the same server (`--tui`, then test `echo`). ink-form is static — there is no picker to hide the alternatives behind — so each branch becomes its own **section**, preceded by a **Variant** select naming which one the call means. The fields in a branch section are rendered optional whatever the branch says: only one alternative applies to a call, so requiring them would build a form that can never be submitted.
+
+The sections are not as independent as they look, which is why the select is not cosmetic: ink-form keeps one value object for the whole form, keyed by field name alone, so two branches both declaring `kind` would be **one** field and the later section's initial value would decide what the earlier one submits. Each branch's fields are therefore rendered under a prefixed name and translated back on submit, where every branch but the chosen one is dropped.
 
 The **CLI** has no form at all, but the same flattening decides how `--tool-arg` values are typed: a branch's `count: { "type": "number" }` is what turns `--tool-arg count=3` into `3` rather than `"3"`. All three read one helper, [`core/json/rootUnion.ts`](./core/json/rootUnion.ts), so they cannot drift on which schemas they can render.
 
-Two things it deliberately does **not** do. A union whose members are not all field-carrying object schemas is left alone rather than offered as a picker with options that render nothing — the schema falls back to whatever its root `properties` describe. And `not` is not interpreted at all: there is no faithful form for "anything except this".
+Three things it deliberately does **not** do, each falling back to whatever the root `properties` describe rather than claiming something untrue. A union whose members are not all field-carrying object schemas is left alone rather than offered as a picker with options that render nothing. A schema carrying **both** `oneOf` and `anyOf` is declined outright: they are independent keywords a value satisfies together, not two spellings of one union, so reading one and dropping the other builds a form that silently omits real constraints — and satisfying both honestly means offering the cross product of their alternatives, which no real schema has yet asked for. And `not` is not interpreted at all: there is no faithful form for "anything except this".
 
 #### Unportable tool schemas